i find i got dirty looks more because people were jealous. like "nice car ... but why are you driving it?"
having said that, when people look at my car, i often don't notice and that might appear to be rude.
but if i ever see a car i really respect on the street (m3, 911 turbo/gt2/gt3, e55) i always maybe a point to give a big thumbs up and tell them they have a great car and in every instance they are really appreciative and often humble.